The aim of this study was to evaluate the clinical characteristics and treatment results of 17 children with cavoatrial tumor extension of Wilms tumor. Of the 360 Wilms tumors diagnosed between 1980 and 2000, 17 patients with intracaval thrombus were identified from the medical records at the pediatric oncology department of Hacettepe University. The following data were collected and reviewed: age, sex, presenting symptoms, turner site, presence of anaplasia, stage, associated congenital anomalies, localization of tumor thrombus, radiologic findings, type and duration of preoperative chemotherapy, response to preoperative chemotherapy, recurrences, and survival. The frequency of cavoatrial extension in this group was 4.7% (15 in the interior vena cava and 2 in the right atrium). Fourteen patients received preoperative chemotherapy consisting of two-drug regimen (vincristine and actinomycin D) ranging from I to 12 weeks (median 4 weeks). Since intravascular invasion is often asymptomatic, a careful radiologic examination to detect tumor thrombus before surgery is essential. There is no need for aggressive Surgery in the presence of tumor thrombus. It may be resolved by preoperative chemotherapy. Surgical removal of the thrombus should be considered in the presence of life-threatening tumor thrombosis at diagnosis and in patients who had residual thrombus after chemotherapy.